Filtration systems using panel filters or cartridge filters employ a porous filter material to filter particulate from a gaseous stream. The removed dust or particulate solid is retained on an up-stream surface of the filter medium as the gas passes through. Many of the environments in which panel or cartridge filters are used must be able to withstand elevated temperatures of approximately 400.degree. F. or higher. Current high efficiency fabric filtration media that operate in this high temperature and corrosive environment consist primarily of two types of filters. One is a fiberglass fabric with a layer of expanded polytetrafluoroethylene (ePTFE) membrane laminated to the fabric surface. The other is a felt constructed of stretched porous polytetrafluoroethylene membrane material. U.S. Pat. No. 4,983,434 to Sassa exemplifies the latter. In the Sassa patent, a polymer adhesive, such as a fluorinated ethylene propylene (FEP) copolymer, is coated onto a felt and a layer of expanded porous polytetrafluoroethylene membrane material, prepared in accordance with the teachings of U.S. Pat. Nos. 4,187,390; 4,110,392; and 3,953,566, is laminated to the FEP containing side.
Both the fiberglass fabric-polytetrafluoroethylene laminate and the expanded porous polytetrafluoroethylene felt filtration laminate are capable of operating in corrosive environments at temperatures of about 500.degree. F., with the latter being more resistant than the fiberglass laminate. However, when these materials are constructed into a pleated cartridge, they must be supported by an internal wire cage with external metal support with stays between each pleat. U.S. Pat. No. 4,878,930 to Manniso, et al., exemplifies the pleated cartridge support system that would be used with the membrane of the Sassa patent. The support taught by the Manniso patent is required because the prior art filter materials are nonrigid in nature and cannot retain a pleated shape without supplemental support. The supplemental support renders the cartridges more expensive, and subject to corrosion at the support site with reduced surface area available for filtering.
Rigid composite polyamide and polyaramid non-woven or felt backed laminates are available, but most laminates are limited to applications involving temperatures below 400.degree. F. The most common variety are non-woven polyester backed laminates which are limited to 260.degree. F. Thus, a need exists for a porous polytetrafluoroethylene membrane laminate that functions as a self-supporting and pleated filter element for lightweight high temperature use, without requiring the cage and support structures utilized in the filter cartridge art.
The laminates of the present invention avoid the drawbacks of the prior art, e.g., avoid the non-rigid nature of the prior art materials, and provides a self-supporting membrane laminate that is capable of withstanding temperatures of up to 500.degree. F. or greater in the presence of harsh chemicals.